QT-prolonging medicine combinations: a guide to risky pairings

Each heartbeat involves the heart muscle contracting and then electrically resetting. The QT interval on an ECG is a measure of that cycle. If the reset phase is prolonged, the heart can be vulnerable to a dangerous rhythm known as torsades de pointes, which may cause fainting or cardiac arrest.

The inherited form is covered in our long QT syndrome guide. This page is about the acquired form, where a medicine, a chemical imbalance or both lengthen the QT in someone whose heart was previously normal.

The NHS names antibiotics, antihistamines, antidepressants and heart medicines. Our fetched NHS pages add detail: citalopram carries a specific heart-rhythm warning; the fluconazole page lists antipsychotics including pimozide, quetiapine, amisulpride, haloperidol and risperidone, escitalopram, erythromycin, clarithromycin and amiodarone among the medicines to check; the clarithromycin page lists domperidone and ivabradine.

Not every medicine in a class affects the QT to the same degree, and the effect can depend on the dose and on the individual. This is why prescribers consult interaction references and do not rely on class names alone.

• Macrolide antibiotics such as clarithromycin and erythromycin.

• Antifungals such as fluconazole.

• Some antidepressants including citalopram and escitalopram.

• Some antipsychotics including haloperidol and quetiapine.

• Some antiarrhythmics such as amiodarone.

• Domperidone for nausea.

There are two main routes. In the first, two medicines each lengthen the QT, so the effects add up. In the second, one medicine slows the clearance of another, so the level of a QT-affecting drug rises. Fluconazole and clarithromycin are known to alter how other medicines are handled, which is why they appear in many interaction lists.

A third factor is the patient: older age, existing heart disease, a slow heart rate, kidney problems and low potassium or magnesium increase vulnerability. A short course of a macrolide antibiotic in someone already taking an antipsychotic and a diuretic is the sort of scenario where a pharmacist review is valuable.

The NHS acquired long QT page says very low potassium, magnesium or calcium can cause the condition and recommends potassium-rich foods or supplements under medical direction when levels are low. Water tablets, vomiting and diarrhoea can reduce potassium, so a gastrointestinal illness is a time to take extra care with a QT-affecting medicine.

This does not mean taking potassium supplements without advice. Too much potassium has its own dangers, covered in our guide to potassium-raising medicine combinations.

Before or during treatment, a prescriber may review the list of medicines, ask about fainting, palpitations or a family history of sudden death, check blood potassium and magnesium, and arrange an ECG when risk is higher. Which of these happens depends on the medicine and the person, and the decision is theirs.

If you have ever been told you have a long QT or have had unexplained fainting, make sure every prescriber and pharmacist knows, including the dental and out-of-hours teams.

Seek urgent help for fainting without clear cause, a racing or irregular heartbeat with dizziness, or a seizure. The NHS says to call 999 if someone is not breathing normally and not responding, and to seek urgent GP care for recovered fainting or repeat palpitations. Do not stop a prescribed medicine suddenly on your own without taking advice, but do make that call quickly.